Cellular communication systems have traditionally used licensed frequency bands, and still use. The 3rd Generation Partnership Project (3GPP) initiative “License Assisted Access” (LAA) intends to allow Long Term Evolution (LTE) equipment to also operate in the unlicensed radio spectrum such as the 5 GHz band. The unlicensed spectrum is used as a complement to the licensed spectrum. Accordingly, devices connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell. It is also possible that also the PCell operates in unlicensed spectrum.
In this context, the primary cell is a cell operating on a frequency on which wireless devices perform initial connection establishment procedures or initiates connection re-establishment procedure, e.g., indicated as primary cell in handover procedure. The term is used to differentiate from secondary cell, which is a cell operating on another frequency which may be configured once a radio resource control connection is established, wherein the secondary cell may be used to provide additional radio resources.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so-called listen-before-talk (LBT) method may need to be applied. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
The term “unlicensed spectrum” is widely used within the field and emanates from that national, regional or international radio spectrum institutions, e.g., Federal Communications Commission (FCC), Electronic Communications Committee (ECC), International Telecommunication Union (ITU), make spectrum available either on a licensed or unlicensed basis. While a licensed spectrum is allocated to a particular entity, i.e., operator or government (military, aviation, marine, etc.), the unlicensed spectrum is made available for any innovator or consumer who can use the unlicensed spectrum just by following some technical rules set up for the unlicensed spectrum. This low-regulation system lets innovators deliver a multitude of unlicensed offerings such as Wi-Fi hotspots, medical equipment, industrial/logistics/inventory systems, wireless headsets, cordless phones, remote car door openers, wireless keyboards, etc. This disclosure relates to wireless communication similar to what is used in cellular communication systems, e.g. (LTE) in licensed spectrum but where also unlicensed spectrum may be used.
The LBT procedure leads to uncertainty at the eNodeB (eNB) regarding whether or not it will be able to transmit one or more downlink (DL) subframes. This leads to a corresponding uncertainty at the user equipment (UE) as to whether or not it actually has a subframe to decode. An analogous uncertainty exists in the UL direction where the eNB is uncertain if the UEs actually transmitted.
Data rate selection by selecting a modulation and coding scheme (MCS) to the UEs is primarily based on channel quality indicator (CQI) reports sent from the UE to the eNB via the PCell in LAA. Of course, data buffer and quality of service (QoS) is also used as input to decide this. While the channel quality is good as indicated by the CQI report, the eNB for other reasons may not able to recently schedule transmission of any discovery reference signal (DRS) for the UEs to perform timing, frequency and gain setting adjustments.
In LAA, eNB must perform listen before talk (LBT) prior to data transmission on the SCell. LBT Category 4 with exponential back-off is a non-aggressive scheme that allows good coexistence with Wi-Fi and other unlicensed spectrum users. The discovery signal that is transmitted every 40 ms or so is an important reference signal transmitted to allow the UE to maintain coarse synchronization with the eNB. It will use a more aggressive LBT mechanism to ensure that it is not starved. Even so, due to the load in the band it cannot be guaranteed that it will always succeed. Current assumption is, that the start of the discovery signal is restricted to LTE subframe borders and that the start of regular data transmissions are restricted to a few fixed positions within the subframe, including the subframe border.
Recently there have also been proposals to operate LTE in unlicensed spectrum without the aid of a licensed carrier. In such an operation, the PCell will also operate on the unlicensed carrier and thus essential control signals and channels will also be subject to unmanaged interference and CCA.
Further the carrier (re)selection process (when the network node changes its carrier frequency during operation) becomes more problematic when it is also applied to the PCell (or serving cell in IDLE), because then there is no cell that the UE is “anchored” to during the carrier frequency change.
In systems such as LTE, system information is provided and may be broadcasted by a network node over a logical channel, e.g., a broadcast control channel (BCCH). This logical channel information may further be carried over a transport channel, e.g., broadcast channel, BCH, or be carried by a downlink shared channel (DL-SCH). There may be two parts in system information: static part and dynamic part. The static part is usually called master information block (MIB), and is transmitted using, e.g., the BCH and may for example be carried by a physical broadcast channel (PBCH) once every 40 ms. The MIB may carry useful information which includes channel bandwidth, physical hybrid automatic repeat request indicator channel (PHICH) configuration details, transmit power, number of antennas and SIB scheduling information transmitted along with other information on the DL-SCH. The dynamic part is usually called system information block (SIB), and may be mapped on radio resource control (RRC) system information messages over DL-SCH and transmitted using physical downlink channel (PDSCH) at periodic intervals. There are different types of SIBs having different tasks and being transmitted with different intervals. They are normally referred to as SIB1, SIB2, etc. For example, SIB1 may be transmitted every 80 ms and provide cell access related parameters and scheduling for other SIBs, SIB2 may be transmitted every 160 ms and provide common and shared channel configuration, and SIB3 may be transmitted every 320 ms and provide parameters required for intra-frequency, inter-frequency, and inter-radio access technology re-selection. SIBs may be grouped in system information containers, where each container may be composed of multiple SIBs.
In LTE Release 12 there is a clear relationship with the position of synchronization signals and the position of the master information block (MIB) and system information block type 1, SIB1, as illustrated in FIG. 13, which are essential to decode quickly for the UE. The secondary synchronization signal with sequence zero (SSS0 in subframe 0, SF0) points out position of the MIB. The same signal with sequence 1 (SSS1 in subframe 5, SF5) points out the position of SIB1.
Later in LTE Release 13, since no system information (MIB and SIB1) is transmitted in the unlicensed SCell, as illustrated in FIG. 14, there is no consideration regarding “floating” and transmitting the secondary synchronization signals SSS0 and SSS1 pretty much in any subframe. The following was agreed in Release 13 LAA: Scrambling sequences of PSS/SSS/CRS/CSI-RS composing DRS are generated using subframe index 0 when transmitted in subframe 0˜4, and using subframe index 5 when transmitted in subframe 5-9.
LTE-U Standalone is based on LTE Release 13 but is required to transmit the system information including MIB and SIB1 in a discovery measurement timing configuration, DMTC, window and must transmit using the rules mentioned above.
Note that also in Release 13 and now in LTE-U Standalone, regular SSS0 and SSS1 will be transmitted opportunistically (piggybacked when there is data to transmit) outside of the DMTC.
Before the UE has read system information, it does not know where in time the DMTC window located and thus it does not know if a detected PSS/SSS pair coincides with a system information transmission. Hence due to SSS signals appearing in various subframes with and without the MIB/SIB1 being present it can confuse the UE, wherein there is a risk of spending power and computing resources in vain. It is therefore a desire to alleviate this problem.